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Since the foundation of 20 years before,
MITS has been manufacturing and selling
Prototyping Machines.
In genaral, there are two types of PCB making machines.
One type is the prototyping machine for trial board manufacturing
and the other is the
mass-production type, each type has different needs and
purposes.
While the mass-production type is larger in size and suitable
for the high quantity
output, the prototyping machine is smaller in size for multi-purpose
use
mostly in the research and development laboratories.
As a manufacturer of prototyping machines, let us explain
the differences and
features of each type of prototyping machine.
The spindle revolution and the proessing speed
The processing
speed is the key element required for the mass-production
model.
And it must also be durable to the heavy loading and the
nonstop operation,
resulting in equipping the large-scale and high-priced spindle
motors.
But prototyping machines are demanded for the miniatuarization
with smaller spindle motor, because they process smaller
amount of PCBs.
When compared with the mass-production type, the compact-sized
spindles
are less durable for heavy load and tend to have more runout
when in
high-speed rotation.
Generally speaking, high-speed rotation is required for
small-diameter drilling.
But with the presence of runout, small diameter drill cracks.
Correlation
of the rotating speed of spindle motor and its runout
Measured by MITS NANOMETRIC SENSOR MODEL 211
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As stated above, for prototyping machines,
the rotating speed of the spindle motor significantly affects
the degree of runout.
Meanwhile, you are able to drill the hole as small as 0.1mm
diameter (even though
the rotating speed is not high) , when you lower the penetrating
speed that causes
the small runout.
Also in milling a narrow path, the machine vibration and
the orthonormality of Z axis,
that is the degree of rigidity, is the most important factor,
when considering the characteristics of milling bit we use
for the machine.
 The
resolution does not directly indicate the processing accuracy
Merely interpreting the processing accuracy as
the resolution is not correct.
The processing accuracy of the prototyping machine is largely
influenced by various
factors such as precision of the lead screw, the backlash,
precision of the stepping
motor of each axis, the runout of the spindle, deflection
of the machinery, the
vibration and the thermal changes, aside from the theoretical
resolution.
Being unable to realize the processing in the minimum resolution
written in the
catalogue can easily be revealed by performing the actual
processing.
I am going to explain the reason why, by taking the precision
of Z-axis as an example.
In processing PCB, what is particularly important is the
evenness of the milling depth.
In case of using the milling bit with 90 degree for example,
both pattern and milling
width vary according to the depth of the milling bit.
Usually, the depth of Z axis is controlled by the computer
command.
As for X-Y 2 axes prototyping machine, Z axis is controllable
only the vertical motion.
As to X-Y-Z 3 axes controllable machine, it is computer-controlled
by stepper motor.
The resolution represents the design value of its controlling
parts (stepper motor and
its screw for driving conductance), not the measured accuracy.
Now, here is the critical problem.
That is, the table in which the machine put can never be
in horizontal position.
When you put the PCB on the setting board, the deviation
of the flatness exceeds
100µm. Processing with the milling bit with angle (pointed-end
milling bit) ends up in causing the uneven pattern width.
(Watch the movie illustrating absolute processing)
Now, you can understand a mere amelioration of the resolution
limit does not make
sense at all under these circumstances.
In actual PCB processing, it is performed by the incremental
processing. It demands
the accurate protrusion of the milling bit with the precise
distance from the PCB, so to speak the accuracy of "four
dimentional control", therefore special attention in
designing must be paid, which is entirely different from
the resolution written in the specification.
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Absolute Milling
As the milling bit is fixed, milling on the
uneven surface of the PCB makes the
milling width also uneven.
>>Watch the movie clip (1825KB)
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What is the Machine Rigidity?
For prototyping machine, the rigidity
is the critical factor.
"High rigidity" directly linked to the fine and precise finishes
of the processed pattern,
such as;
- Milling path is fine and precise.
- Milling surface is fine and precise just like
being cut by a keen edge.
- Milling path is even.
- Milling pattern is processed precisely as you
designed by CAD.
Can we represent the rigidity by numerical value?
The answer is, unfortunately, no.
Then what is the criteria in selecting the machine?
The professionals in selecting the machine distinguish them by the
following two factors.
Firstly, they estimate the overall designing technology and the
components constitutes
the machine.
We call it "static rigidity"
Secondly, they assess "dynamic rigidity"
Dynamic rigidity emerges as vibration, swinging, and deflection.
Instead of examining the those critical factors of the machine,
if you make a decision by
budged-pleasing price, mere comparison of specification or its style,
you may end up in
undesirable processing of PCB by an inappropriate prototyping machine.
Higher Frequency Circuit and Trial Sample
board
Recently, the high frequency circuits are in great demand in the
electronic devices
as following.
- Wireless LAN
- Security IC Tags
- Defense and security equipment
- Satellite transmission
- In-vehicle unit
- Medical examination equipments
In order to streamline the designing, the circuit
simulation of the software have remarkably
developped, the trial production can be frequently curtailed. But
as for high-frequency circuit,
the demand of the experimental production and the performance assessment
study are
still increasing.
With the ever increasing miniaturization of the electric devices
and utilization of the high
frequency, the high-density technology has advanced and become ever
thinner, resulting in
generating a large number of PCBs consist of less than 100µm
pattern width.
Let us describe a few points at issue in the process from designing
pattern, simulation to
processing the PCB.
DXF Data and Closed-Loop
One of the methods easily enables us to draw the high-frequency
circuit is CAD.
The data drawn by this type of CAD file is outputted by DXF file.
In order to input the DXF data into the prototyping machine and
the other simulation
software, they cannot be accepted unless they are closed-loop.
The example of non-closed loop is as illustrated below;
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CLOSED-LOOP
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OVERFLOW OF LINE
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EXTRA LINES
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COMING IN SUCCESSION OF
CLOSED-LOOP
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BECOMING INTERRUPTED
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The above figures are drawn in an exaggerated form.
Practically, you cannot be aware of the
errors without enlarging it.
It can become the uncertain source of troubles because its input
is not acceptable.
In case of the complicated figure, there are many cases that dozens
of errors should be modified.
Our Challenge to Under 100µm Milling
In the PCB prototyping machine, the limit resolution has long been
regarded as 100µm
Laser beam machining is required for under 100µm line and space
processing.
But laser processing machine is extremely expensive and demands
the facility equipment
or the study of complicated ins and outs, therefore it cannot be
handle "easily" in the laboratory.
Our goal has long been "User-friendly in laboratory". MITS has applied
our technical knowledge
cultivated over the years and successfully achieved the process
smaller than 100µm.
High Frequency Milling bit
In high-frequency field, the pattern figure has to be 90 degree
of rising edge, you need to use
the stub-shaped milling bit.The front edge of milling bit smaller
than 100µm is as thin as human
hair, the know-how of processing differrent from the V-shaped milling
bit is required.
"100µm" is not equal to 100µm?
You may wonder what the above sentence means.
In fact, the milling bit of 100µm cannot process the milling
width of 100µm.
Tool Tolerance
Milling bit supplied by parts manufacturer inevitably come with
some tolerance.
You need to process PCB with putting that tolerance into consideration.
Runout of HF Spindle
The spindle motor in prototyping machine also inevitably come with
some runout.
The runout changes along with the motor rotating speed, and slightly
changes with the extent
of fastening the motor to the collet. These minute change emerges
as the unexpected broad
milling track wider than the milling bit in finer processing than
100µm.
For the 100-micron milling
In high-frequency circuit board, it is indispensable to process
the PCB with plus or minus 5%
precision in 100µm milling, such as Coplanar waveguide.
So as to achieve that resolution, the processing method considering
the tolerance of the milling
bit and the runout of the spindle are abosolutely neccessary.
| Examples: |
When tool tolelance is
+/- 5µm,
and
When runout of the spindle is 10µm. |
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The minimum processible milling width is 105µm,
and that of maximum is 115µm.
Therefore, in order to achieve the 100µm processing, we perform
the following procedure;
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1;
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Use the milling bit from 80 to
90µm considering the tolerance of the milling bit and
the runout of the spindle motor. |
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2;
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Use the milling bit thinner than
80µm and mill the PCB twice to achieve the 100µm
milling width. |
For the fine and precise milling track
Stepping Milling Method
As the milling line gets thinner, processing condition becomes ever
more crucial.
When the radius of milling bit becomes lower than 100µm, the
milling bits become very
fragile, leading to increase burrs.
Therefore, in processing the milling bit thinner than 100µm,
we employ the "stepping method"
that process the PCB gradually, which is suitable for the small
radial milling bit.
In using the copper foil of 18µm thickness, we mill the board
graually by several steps .
(The depth of single milling is 5µm) With this method, you
can process the PCB without
burrs or breakage even if you use the small radial milling bit.
>>Watch the movie
clip (1474KB)
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